Solar systems warped by interstellar wind

Shapes previously attributed to undiscovered planets, star encounters

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The inner, yellow portion of HD 61005's disk spans 5.4 billion miles, or about the width of Neptune's orbit in our own solar system. The outer portions are warped by the interstellar wind. This false-color Hubble view masks the star's direct light to bring out detail in the disk.

Close encounters with interstellar gas could have given the dust-filled disks of solar systems — where planets are thought to form — the odd shapes that some of them have taken on, a new study suggests.

Stars across the galaxy have disks of dusty debris generated by the collisions of small comet- and asteroid-like bodies orbiting each star.

Astronomers have noticed that many of these debris disks are a bit wonky-looking, with lobes of dust sticking out in odd directions. One team noticed just such an oddly-shaped disk while using the Hubble Space Telescope to investigate the composition of the dust around the star HD 32297, which lies 340 light-years away from Earth in the constellation Orion.

John Debes of NASA's Goddard Space Flight Center in Greenbelt, Md., noticed that the interior portion of this star's dusty disk — a region comparable to the size of our own solar system — was warped in a way that was similar to other distant star systems.

Astronomers have previously attributed these warped shapes to the presence of undiscovered planets or past encounters with another star. But Debes and his colleagues used a model to show that the odd shapes aren't likely due to one of these exotic factors, but instead are likely caused by the interstellar environment that the star and its attendant disk are moving through.

"It's important to consider the ecology of these debris disks before running to such conclusions, and this model explains a lot of the weirdly shaped disks we see," Debes said.

Interstellar breeze
The model, described in the Sept. 1 issue of the Astrophysical Journal, simulates the movement of a star and its dust disk as they orbit the center of the galaxy. As it moves through the galaxy, it encounters patches of thin gas clouds that litter the space between stars and create an effect like an interstellar breeze.

"The small particles slam into the flow, slow down and gradually bend from their original trajectories to follow it," Debes said.

The resulting shapes of the dust disk depend on the orientation of the star system to the gas cloud. If the system hits the cloud face-on, which seems to be the case for the star HD 61005 in the constellation Puppis, the disk bends gently away from the direction of motion. Fine dust trails behind, forming a cylindrical wake.

If, on the other hand, the dust disk slices edgewise through the gas cloud, the headwind blows the fine dust away from the portion of the disk inside the cloud, resulting in a lopsided disk.

"The drag from interstellar gas only affects the outskirts of the disk, where the star's gravity can't really hold onto the material," said study team member Alycia Weinberger at the Carnegie Institution of Washington, headquartered in Washington, D.C.

There was also evidence from previous studies that interstellar gas lurked in the region around HD 32297.

"The pieces came together to make me think that gas drag was a good explanation for what was going on," Debes said.

The systems studied are about 100 million years old and resemble our own solar system shortly after the major planets formed. Although astronomers don't know whether planets lurk within the disks of these systems, a better understanding of processes affecting a disk's outer regions will shed light on how "ice giant" planets like Uranus and Neptune — and the more distant swarm of small, icy bodies known as the Kuiper Belt — formed within the solar system.